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Interpreting Viscosity Curves

As a response to the Fill Time, Viscosity & Molding Economics blog I received this question…

E-Mailer
With respect to a in-mold rheology test, why should we choose to process on the right-hand side of the curve?
My Response
As the shear rate, or flow rate, of the polymer increases… the viscosity decreases. This rheological behavior is unique to polymers and is called ‘shear thinning’.
When graphing this, viscosity is plotted on the vertical, ‘Y axis’ and shear rate is plotted on the horizontal, ‘X axis’. Shear thinning will appear as a steep decline in the viscosity of the polymer as the shear rate increases.
Once most of the shear thinning occurs the polymer’s viscosity starts to level out. After this point, the viscosity will remain relatively consistent – resulting in a more stable process. For this reason, you should process on the right hand side of the curve.
Additional Thoughts
You can learn more about this test and other aspects of scientific molding if you participate in our free online webinars.
-Andy

Using Hold Pressure Velocity

I received this question from a frequent blog contributor, and believe it makes a good discussion on decoupled processes…

Milan
On some of our machines, we can adjust the hold velocity. How should this feature be used when molding a decoupled II process.
My Response
Before I respond, I want to relate the general attributes of this process:
  • A process that uses one injection speed to fill – whenever possible
  • The mold fills 95 to 98 percent full during first stage
  • All cavities are short shot during first stage
  • First stage fill is velocity-controlled and not pressure limited
  • Second stage pack is pressure-controller and not velocity limited
  • Process uses only 20 to 80 percent of the machine’s available shot size
  • The final cushion is approximately 10 percent of the overall shot size

In essence, the purpose of a DII process is to ‘decouple’, or separate, velocity controlled fill from pressure controlled pack. To do this, you should either turn the velocity control off so the pressure control takes prominence.

Additional Thoughts
Always make sure you understand the specific controls on your machines. In some cases, the 2nd stage velocity is actually a limit rather than an actual setting. In this case, you should set the speed near the maximum so that 2nd stage pack does not become velocity-limited.
-Andy

Fill Time, Viscosity & Molding Economics

A friend emailed me this great question the other day regarding the relationship between fill speed and pressure…
JM
The pressure required to fill a particular mold went up from 950 psi at 1 in/s to 1050 psi at 3 in/s. 

I understand the higher pressure needed to hit the higher speeds, but shouldn’t this be offset by the significantly reduced viscosity?

Does it hold true that anytime you increase the fill velocity, you see an increase in fill pressure… all things being equal?
My Response
The pressure required to fill will increase because the viscosity change will not completely outweigh the pressure losses… The overall energy consumption does drop considerably during fill since the pressure to fill is being appllied for only one third the amount of time!
It is very likely that a rheology curve would demonstrate that the 1 in/s fill is on the left hand side of the shear thinning transition region and the 3 in/s is on the right hand side of this region.
Think of it in vehicular terms… Your 1 in/s is like pushing a large pickup (higher resistance to movement) at 20 miles per hour, while your 3 in/s is like pushing a compact car (lower ristance to movement) at 60 miles per hour. It takes more gas per minute to move the smaller car at the higher rate, but it takes less time and you consume less gas overall getting to your final destination.
Additional Thoughts
Always keep in mind…the reduced viscosity due to shear thinning will actually save you money… making your processes much more economical since the overall energy consumption to fill the mold as well as the time are reduced.
Additionally, the drop in viscosity will also reduce the pressure required to pack out the mold cavity during 2nd stage. In the long run, all these changes can make a big difference in the productivity and efficiency of your facility.
-Andy

PET Screw Design For Injection Molding

I received this blog topic request yesterday…

Samer
Concerning injection molding, is there a specialized screw used for PET?
My Response
Yes, the goal in PET processing is to (1) maximize output, (2) control shear heat, and (3) minimize melt tempertaure… yet ensure the polymer is fully melted and as homogeneous as possible.
(1) The need to maximize output is due to the fact that most PET injection molding processes are for high-cavitation high-speed molding PET preform machines.
(2) The need to control shear heat is due to the fact that a higher temperature polymer requires more time to cool… and may increase the development of semi-crystalline regions. Also, inconsistent melt temperatures will result in inconsistent cooling and crystallinity… resulting in profit losses due to scrap and troubleshooting.
(3) PET also has very poor thermal conductivity… making it great for drink containers, but terrible for part cooling. The goal in processing PET is to process the material as close to the melting point as possible, while ensuring all the pellets are melted and mixed.
To address all these concerns, different machine manufacturers have devised some great machine and screw designs.
Whether it is a reciprocating screw machine, or a two stage ‘shot-pot’ machine, most PET single screw designs incorporate a barrier screw. In such a design, the screw channel depth remains the same from the feed into the transition zone, but the channel width becomes progressively smaller. As this channel width decreases… an adjacent shallow channel, separated by a barrier flight begins. Ultimately, the melted material flows over the barrier flight into the shallow channel until it transitions into the metering zone. Such a design reduces shear heating and allows stingy pellets more time to melt.
In most single screw designs, a distributive mixing element is typically incorporated on the screw or a static mixer is placed within the nozzle to ensure a more homogeneous melt… especially if a colorant is added to the polymer.
Some of the more ambitious machine manufacturers are working with twin screw designs for two stage molding machines. These systems show great promise since a good twin screw design would provide the best control over both shear and melt temperature.
Additional Thoughts
PET processing is one of the more advanced and refined fields of injection molding, making it very competitive. I strongly suggest anyone getting into PET molding do some significant research to ensure your strategy is well suited to take advantage of today’s advances.
-Andy

The Pitfalls Of Classroom Training

I was recently asked this interesting question…

Customer
I see that you teach at a university, are you planning on providing classroom training?
My Response
Classroom training is not the most effective method available for training your production employees. Some seminars provide 4-8 hours of training a day… which is far too much information to absorb in one week…never mind in one day!
Lengthy classroom training sessions tends to be very disruptive to the production environment. Sending employees out for training can often cost 1-2 times more than the training itself. Likewise, sending a consultant to your plant for on-site training classroom training can be both costly and distracting to your employees. In either case, you still have the underlying issue where the employees do not learn well in the classroom environment.
As an adjunct professor at the Plastics Engineering Department of the University of Massachusetts Lowell, I have successfully blended labs, interactive training, simulation software, product testing, new technology, and outside exercises. Since these classes are comprised of plastics professionals, this helps ensure my participants actually learn the information and understand how it applies to their role as production employees. At Routsis Associates, I have also used this approach successfully in distance learning situations using a variety of media such as interactive training, on-the-job exercises, testing, and short 1-hour training sessions.
Additional Thoughts
As I blogged previously, ask yourself… ‘What Do You Hope To Achieve Through Training?’. To paraphrase… training should be purchased to help meet and sustain specific employee development and competency goals. Keeping this in mind will help you evaluate whether a training method develop competent employees, or just give them a bunch of information.
-Andy

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